Rotating polygon scanning mirrors are typically used in laser printers to provide a “raster” scan of the image of a laser light source across a moving photosensitive medium, such as a rotating drum. Such a system requires that the rotation of the photosensitive drum and the rotating polygon mirror be synchronized so that the beam of light (laser beam) sweeps or scans across the rotating drum in one direction as a facet of the polygon mirror rotates past the laser beam. The next facet of the rotating polygon mirror generates a similar scan or sweep which also traverses the rotating photosensitive drum but provides an image line that is spaced or displaced from the previous image line.
There have also been prior art efforts to use a less expensive flat mirror with a single reflective surface to provide a scanning beam. For example, a dual axis or single axis scanning mirror may be used to generate the beam sweep or scan instead of a rotating polygon mirror. The rotating photosensitive drum and the scanning mirror are synchronized as the drum rotates in a forward direction to produce a printed image line on the medium that is at right angles or orthogonal with the beam scan or sweep generated by the pivoting mirror.
However, with the single axis mirrors, the return sweep will traverse a trajectory on the moving photosensitive drum that is at an angle with the printed image line resulting from the previous or forward sweep. Consequently, use of a single axis resonant mirror, according to the prior art, required that the modulation of the reflected light beam be interrupted as the mirror completed the return sweep or cycle, and then turned on again as the beam starts scanning in the original direction. Using only one of the sweep directions of the mirror, of course, reduces the print speed. Therefore, to effectively use an inexpensive scanning mirror to provide bi-directional printing, the prior art typically required that the beam scans moved in a direction perpendicular to the scan such that the sweep of the mirror in each direction generates images on a moving or rotating photosensitive drum that are always parallel. This continuous perpendicular adjustment is preferably accomplished by the use of a dual axis torsional mirror, but could be accomplished by using a pair of single axis torsional mirrors. It has been discovered, however, at today's high print speeds both forward and reverse sweeps of a single axis mirror may be used, and that no orthogonal adjustment is necessary.
Texas Instruments presently manufactures torsional dual axis and single axis pivoting MEMS devices fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 100–115 microns. The devices include a functional surface, such as a reflective surface or mirror. The dual axis layout may, for example, consist of a mirror supported on a gimbal frame by two silicon torsional hinges, whereas for a single axis device the mirror, or other functional surface, is supported directly by a pair of torsional hinges. The functional surface may be of any desired shape, although when the functional surface is a mirror, an elliptical shape having a long axis of about 4.6 millimeters and a short axis of about 1.5 millimeters is particularly useful. Such an elongated ellipse-shaped mirror is matched to the shape at which the angle of a light beam is received. The gimbal frame used by the dual axis device is attached to a support frame by another set of torsional hinges. These mirrors manufactured by Texas Instruments are particularly suitable for use as the scanning engine for high-speed laser printers and visual display. These high-speed mirrors are also suitable for use as high-speed optical switches in communication systems. One example of a dual axis torsional hinged mirror is disclosed in U.S. Pat. No. 6,295,154 entitled “Optical Switching Apparatus” and was assigned to the same assignee on the present invention.
According to the prior art, torsional hinge devices having a mirror as the functional surface were initially driven directly by magnetic coils interacting with small magnets mounted on the pivoting mirror at a location orthogonal to and away from the pivoting axis to oscillate the mirror or create the sweeping movement of the beam. In a similar manner, orthogonal movement of the beam sweep was also controlled by magnetic coils interacting with magnets mounted on the gimbals frame at a location orthogonal to the axis used to pivot the gimbals frame.
According to the earlier prior art, the magnetic coils controlling the mirror or reflective surface portion typically received an alternating positive and negative signal at a frequency suitable for oscillating the device at the desired rate. Little or no consideration was given to the resonant pivoting frequency of the device. Consequently, depending on the desired oscillating frequency or rate and the natural resonant frequency of the device about the pair of torsional hinges, significant energy could be required to pivot the device and especially to maintain the device in a state of oscillation. Furthermore, the magnets mounted on the functional surface of the device portion added mass and limited the oscillating speed.
Later torsional devices having a mirror as the functional surface were manufactured to have a specific resonant frequency substantially equivalent to the desired oscillation rate for applications where the mirror apparatus was used as the scanning engine. Various inertially coupled drive techniques including the use of piezoelectric devices and electrostatic devices have been used to initiate and keep the mirror oscillations at the resonant frequency.
It has now been discovered that the earlier inexpensive and dependable magnetic drive can also be used and set up in such a way to maintain the pivoting device at its resonant frequency or to provide orthogonal motion. Unfortunately, the added mass of the magnets becomes more and more of a problem as the required frequency increases to meet higher and higher operational speeds. Further, as mentioned above, although the reflecting surface of a scanning mirror can be of almost any shape, including square, round, elliptical, etc., an elongated elliptical shape has been found to be particularly suitable. Unfortunately, such elongated elliptical-shaped devices, such as a mirror, introduce inertia forces that result in excess flexing and bending of the functional surface of the device adjacent the hinges and tips of the device such that if the functional surface is a mirror, the mirror no longer meets the required “flatness” specifications for providing a satisfactory laser beam. The thickness of the device may be increased to maintain the necessary flatness, but the added weight and mass results in excess stress on the torsional hinges, which can cause failures and/or reduced life.
Therefore, a scanning device, such as a mirror, having both a low mass moment and sufficient stiffness to maintain acceptable flatness at high oscillation speeds would be advantageous.